Steam cracking of ethane to form lower olefins such as ethylene and propylene provides undesirable low value dienes (diolefins) in pygas as well. Diolefins are extremely reactive, unstable, and difficult to process in conventional refinery units. For example, the reactive di-olefins can form gum which plugs conventional hydrotreating beds as well as plugging upstream heat exchangers or heaters. Accordingly, refiners have often had to resort to extreme and expensive measures to process these materials, e.g., high pressure hydrotreating. Catalytic diolefin conversion upstream of conventional hydrotreating is also possible, e.g., UOP, Inc.'s Platfining process, which operates at temperatures low enough to prevent gum formation. U.S. Pat. No. 4,097,367 discloses upgrading of pyrolysis gasoline from steam cracking to make ethylene, by passing naphtha over Pd/Zn/ZSM-5 at 900.degree. to 1200.degree. F. High temperature processing of C5+ fraction converted everything boiling in the BTX range to aromatics, yielding a liquid product with essentially no non-aromatic hydrocarbons boiling above 167.degree. F. U.S. Pat. No. 5,053,579 discloses a process for upgrading unstable olefins, naphthas and dienes, such as coker naphthas, by oligomerizing over shape selective zeolite to gasoline and distillate products. U.S. Pat. No. 4,751,339 discloses a process for upgrading diene-containing olefins to aromatic hydrocarbons by contacting a feedstock of liquid pyrolysis gas byproduct under high severity conditions in a fluidized bed of acidic zeolite catalyst particles, e.g., ZSM-5 in the presence of C3+ alkanes. Aromatics yield is increased by recovering and recycling to the reactor C5+ aliphatic hydrocarbons.
U.S. Pat. No. 4,088,706 describes converting methanol to a mixture of C.sub.2 to C.sub.3 olefins and mononuclear aromatics, particularly p-xylene, by contacting with a shape-selective zeolite having a Constraint Index of 1 to 12 which has been modified by adding oxide of boron or magnesium either alone or in combination or in further combination with oxide of phosphorus. U.S. Pat. Nos. 4,049,573 and 4,088,706 disclose modifying zeolites, such as ZSM-5 with oxides of boron or magnesium, either alone or in combination or in further combination with oxide of phosphorus, increases the yield of p-xylene in the catalytic conversion of methanol to olefins and aromatics. U.S. Pat. No. 4,480,145 discloses increasing ethylene yield in the catalytic conversion of methanol over ZSM-5 by moderating diffusivity of the zeolite by use of large crystal form of the zeolite and by silica "stuffing" of the zeolite pores. U.S. patent application Ser. No. 09/055,478 (Docket No. 10017), filed Apr. 6, 1998, which is a continuation-in-part of U.S patent application Ser. No. 08/725,277, filed Oct. 2, 1996 discloses a catalyst for converting methanol to C.sub.2 to C.sub.4 olefins having a Diffusion Parameter for 2,2-dimethylbutane of 0.1-20 sec.sup.-1 such as phosphorus oxide-containing ZSM-5.
None of the prior art processes describe a way to efficiently convert a mixture of oxygenates and unstable diolefins to high value petrochemicals, e.g., ethylene, propylene, benzene, toluene and xylenes.
It would be desirable to provide a way to efficiently convert unstable by-products of steam cracking to higher value hydrocarbons. It would also be desirable to convert unstable diolefin products to higher value aromatic hydrocarbons such as para-dialkylbenzene products such as p-xylene, as well as to the desired lower olefin products of steam cracking. Inasmuch as steam crackers are built to produce ethylene and propylene, a conversion process for diolefins which provides such low molecular weight monoolefins as necessary by-products would be highly efficient for converting diolefins produced during steam cracking. A steam-cracking facility could thus readily increase its capacity for producing ethylene and propylene by employing a method which would simultaneously upgrade pygas dienes to higher value products such as p-xylene, as well as desired incremental ethylene and propylene.